In a typical cathode ray tube display of the type commonly used for computer monitors and the like, an electron gun assembly produces one or more electron beams which are directed toward a phosphor coated imaging screen. In monochrome monitors, a single electron beam is produced while in most color monitors a plurality of electron beams are produced. The electron beam is focused to the desired spot size at the CRT phosphor screen and accelerated by a high voltage accelerating potential within the cathode ray tube to cause the electrons within the electron beam to impact the phosphor areas of the phosphor screen with sufficient energy to produce the desired light output. In addition, the electron beam or beams are deflected both horizontally and vertically with respect to the phosphor screen to produce a swept raster in which the screen is scanned in one direction at a higher scan rate and progressively scanned in the orthogonal direction at a somewhat slower rate. In most display systems, the horizontal scan occurs at a much higher scan rate than the vertical scan.
The desired image is formed upon the phosphor screen by intensity modulating the electron beam or beams in accordance with an image signal. To properly display the image, the scanning process must be synchronized to the image signal. This function is generally accomplished by including a plurality of vertical and horizontal scan synchronizing signals within the image signal in what is often referred to as the composite video and sync signal.
In many situations in which computer monitor displays are utilized, it is desirable to operate the display scanning systems at a plurality of scan frequencies. Because the systems which produce the cathode ray tube scanning and high voltage electron beam accelerating potential are highly interactive and frequently utilize frequency tuned circuit components, changes of scan frequency produce undesired and often damaging responses within the interrelated system components. This is particularly true of the horizontal or higher scan rate deflection portion of the display scanning system. To accommodate the interactive changes within the display system and provide appropriate compensation to avoid performance loss or component damage, practitioners in the art have generally attempted to provide switching systems which introduce compensating changes of component values into the scanning and electron beam accelerating potential generation system when scan frequency is changed.
For the most part, such systems have generally involved the use of a plurality of switches together with redundant components. In other systems, large segments of the display scanning system are provided in a virtual complete redundancy to accommodate frequency changes. Such systems have been found to greatly increase the monitor costs and often introduce additional sources of unreliability to the overall monitor performance. There remains, therefore, a need in the art for an improved scan compensation system for use within multiple scan frequency monitors.
Accordingly, it is a general object of the present invention to provide an improved multiple frequency scan system for display monitors or the like. It is more particular object of the present invention to provide an improved multiple frequency scan system which automatically compensates for changes of scan frequencies to maintain consistent monitor performance.